In a network having a plurality of options for a packet transfer route such as ad hoc networks, when a failure has occurred while a packet is being relayed on an optimum route that was originally selected, the packet is transferred to an alternative route.
In a communication network as illustrated in FIG. 1 for example, when a data packet is to be transferred from a node 101 to a node 105, there are three transfer routes, i.e., routes via nodes 102, 103, and 104, respectively. First, the transfer route via the node 102 is selected, and when the packet transmission from the node 101 to the node 102 has failed, the transfer route via the node 103 is selected so that the data packet is transferred from the node 101 to the node 103 as an indirect transfer.
However, when the packet transmission from the node 101 to the node 103 has also failed, the transfer route via the node 104 is selected so that the data packet is transferred from the node 101 to the node 104 as an indirect transfer. When the packet transmission from the node 101 to the node 104 has succeeded, the data packet is transmitted from the node 104 to the node 105 and the data packet reaches the node 105.
As described above, by providing an indirect transfer function to a communication network, the reachability with which data packets reach their transmission destinations can be increased even when the route data does not represent the actual condition of the communication network completely.
However, in some cases, indirect transfer functions worsen situations when there is congestion in networks. In a communication network as illustrated in FIG. 2, for example, when a data packet is to be transferred from the node 201 to the node 206, there is a route via the node 205 and a route via the nodes 203 and 204 between nodes 202 and 206.
When the transfer route via the node 205 is first selected so that the data packet is transmitted from the node 202 to the node 205, an acknowledge response (Ack) packet 211 is transmitted in return from the node 205 to the node 202. Thereafter, the data packet is transmitted from the node 205 to the node 206 so that the data packet reaches the node 206.
When the returning of the Ack packet 211 is prevented by congestion, the Ack packet 211 does not reach the node 202, and this causes an indirect transfer in which the same data packet is transferred as a duplicate packet 212 to the transfer route via the nodes 203 and 204. Thereafter, the duplicate packet 212 is transmitted from the node 204 to the node 206 so that the duplicate packet 212 reaches the node 206. No problems arise even when the node 206 receives the same data packet a plurality of times; however, repeated transmission of many duplicate packets worsens the congestion of the network.
In order to suppress duplication of data packets, methods of transmitting an Ack as described below are proposed.
(1) A method in which the transmission of an Ack is multiplexed
In this method, when a data packet is transmitted from a transmitting node to a receiving node, an Ack packet is transmitted in return from the receiving node to the transmitting node a plurality of times. Thereby, the reachability of an Ack packet increases, suppressing duplication of data packets.
(2) A method in which transmission confirmation of an Ack is performed
In this method, when a data packet is transmitted from a transmitting node to a receiving node, a first Ack packet is transmitted in return from the receiving node to the transmitting node, and subsequently a second Ack packet, indicating the reception of the first Ack packet, is transmitted in return from the transmitting node to the receiving node. When the receiving node does not receive the second Ack packet, the first Ack packet is retransmitted from the receiving node to the transmitting node. Thereby, the reachability of first Ack packets increases, suppressing duplication of data packets.
(3) A method in which the transmission power of an Ack is adjusted
In this method, when a data packet is transmitted from a transmitting node to a receiving node, an Ack packet is transmitted in return with increased transmission power from the receiving node to the transmitting node. Thereby, the reachability of first Ack packets increases, suppressing duplication of data packets.
A method is known in which the link quality is evaluated by calculating the transmission success rate between nodes included in a communication network in order to detect congestion in the communication network. However, it is difficult to determine a success and failure of transmission on the basis of measurement in the receiving node alone unless the number of times of transmitting data packets and Ack packets and their transmission timings are known, because they are transmitted unperiodically. Accordingly, the transmission success rate is calculated on the basis of the fluctuation of arrival cycles of Hello packets, which are transmitted periodically.
A case is discussed as an example in which the intervals at which one node receives the first through ninth Hello packets are measured and the measurement results as illustrated in FIG. 3 are obtained. In this example, the second and fifth reception intervals are 2T and 3T, respectively, which are longer than other reception intervals. In such a case, it can be estimated that missing Hello packets due to transmission failures have elongated the reception intervals, and accordingly the transmission success rate can be calculated on the basis of the measurement results of the reception intervals.
A technique is also known in which a wireless data communication system makes the transmission device record transmission time and the transmitted contents and also makes the reception device record the reception time and the received contents so that the recorded contents are analyzed in order to determine whether or not the transmission device and the reception device were operating normally when a failure occurred in communication.
A technique is also known in which, in a mobile communication network, a transmitting node transmits a measurement packet stream periodically and a receiving node transmits the measurement values of packet delay to the transmitting node so that the state information of each route is updated.
A technique is also known in which, in a mobile ad hoc network, link quality is measured on the basis of a packet error rate in order to determine a route on the basis of the measurement result.
Patent Document 1: Japanese Laid-open Patent Publication No. 9-284236
Patent Document 2: Japanese Laid-open Patent Publication No. 2005-210671
Patent Document 3: Japanese Laid-open Patent Publication No. 2005-535235